Macrophage endoplasmic reticulum (ER) stress in regulating the pathogenesis of atherosclerosis is not fully understood. The central goal of this proposal is to examine the role of ER-associated protein Nogo-B in atherosclerosis. We have shown that Nogo- B is highly abundant in endothelial cells (EC), vascular smooth muscle cells (VSMC) and monocytes/macrophages with diverse and cell-specific function. The genetic loss of Nogo-B (Nogo-/-) results in exaggerated neointima formation after arterial injury. The expression of Nogo-B negatively correlates with the severity of atherosclerosis, suggests that the local reduction of Nogo-B might contribute to plaque formation and/or instability. Our exciting preliminary data demonstrate that Nogo-/- mice on an ApoE knockout background (ApoE-/-Nogo-/-) develop larger and more advanced atherosclerotic lesions with increased macrophage apoptosis compared to ApoE-/- mice. The ApoE-/-Nogo-/- mice also developed severe coronary stenosis, a phenotype that is rarely observed in current mouse models of atherosclerosis. Nogo-/- macrophages are much more prone to apoptosis in response to free cholesterol loading compared to those of wild-type mice. We also show that Nogo-B can be secreted and transferred from ECs to macrophages in vitro and in vivo, and that exogenous Nogo-B can prevent macrophage apoptosis. We hypothesize that Nogo-B governs macrophage functions to regulate atherosclerotic plaque formation and necrosis. In this proposal, we will: (1) define the role of macrophage Nogo-B in atherosclerotic plaque progression and necrosis in vivo, (2) interrogate the mechanisms by which cell intrinsic Nogo-B regulates ER stress induced macrophage apoptosis in vitro, and (3) examine the non-cell- autonomous effects of Nogo-B in macrophage function in vitro and atherogenesis in vivo. The findings of this study will advance the Nogo field, providing insights in understanding how ER-associated reticulon proteins regulate vascular homeostasis.